1. Field of the Invention
The present invention relates generally to a method for manufacturing an array of nanoclusters and a substrate with an ordered array of nanoclusters. More particularly, the method deposits atoms upon a surface containing an ordered array of nanoscale holes that have been produced by etching a surface patterned by a mask containing a regular array of nanoscale pores.
2. Description of Related Art
The ability to control function by controlling size makes nanoclusters very attractive for technological applications in high-speed computing, high density data storage and display, and optical communications through devices such as the single-electron transistor and the quantum dot laser. Designs for such devices require not only sharp control of nanocluster size, but also fabrication of ordered arrays of nanoclusters and, in some cases, interconnections between clusters within the array.
As has been discussed elsewhere (for example, J.-M. Gerard 1995), single layer quantum dot arrays have been demonstrated to have excellent optical properties such as high quantum efficiency, long radiative lifetimes, and very fast PL rise times. However, direct growth has been stymied by xe2x80x9cthe prerequisite of an ultrafine lithographic definition of the mask.xe2x80x9d
Dramatic advances have been made recently in obtaining ordered arrays of nanoclusters from liquid phase syntheses by selective precipitation and Langmuir-Blodgett techniques Murray et al. (1993) Ohara et al. (1995) Murray et al. 1995, Whetten et al. 1996; Luedtke et al. (1996); Heath et al. 1997. Ordered arrays have also been produced using films of close-packed polystyrene spheres as deposition masks [Hulteen et al. (1995).] Ensembles of individual, size-controlled InP quantum dots grown by self-assembly in molecular beam epitaxy on a GaAs surface have emitted light of very narrow bandwidth at a wavelength determined by the size of the dots [Grundmann et al. (1995)]; embedded between electron-injecting and hole-injecting layers, these dots have exhibited lasing [Kirstaedter (1996)]. However, because they grow at randomly distributed nucleation sites on the substrate, their location is difficult to control.
From the point of view of device fabrication, it is desirable to first define the desired nanoscale array pattern directly on the substrate and then grow or deposit the nanoclusters on the patterned substrate. The nanoclusters produced preferably have diameters less than about 25 nm to show true quantum confinement behavior.
In earlier work, Heath and co-workers [1996] studied the formation of clusters in confined geometries by defining 100 and 150 nm diameter holes in a thin oxide mask over a Si wafer and then growing Ge clusters on the Si surfaces exposed in the etched holes Gills et al. (1992). They observed a few clusters in each 150 nm hole at locations distributed over the bottom of the hole. A few of the 100 nm holes contained a single cluster, but difficulties with that sample precluded complete analysis. Their results showed that the confining geometry of the 150 nm hole limited the number and size of clusters growing in the hole but did not precisely control their location.
What was needed, and what was apparently not available until the presently described invention, is a method of controlling the position as well as the size of a nanocluster. By etching holes an order of magnitude smaller in diameter than those of Heath and co-workers, we observe the formation of a single nanocluster in each hole when Ti adatoms are deposited on a Si substrate that has been etched to define an array of nanometer-sized holes. The symmetry and lattice constant of the array (as determined by atomic force microscopy (AFM)) are identical to those of the etched holes, demonstrating that these extremely small holes control the position as well as the number of clusters grown in each hole.
It is an object of the present invention to provide new nanopattern mask materials which allow formation of nanoclusters without the slow throughput of electron beam lithography and the high cost of X-ray lithography.
It is another object of the present invention to provide new nanopattern masks which intrinsically contain mesoscopic scale openings.
It is a further object of the present invention to provide a process for creating nanoclusters combining the steps of obtaining a biologically derived mask, transferring the mask pattern to a substrate using low-damage dry etching, and initiating cluster growth by adatom deposition.
It still another object of the present invention to achieve massively parallel processing in fabricating an ordered and precisely positioned array of nanoclusters.
It is yet another object of the present invention to create arrays of holes having diameters small enough to induce the formation of nanoclusters which exhibit quantum confinement behavior without causing adjacent lattice damage to the substrate.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention broadly described herein, one embodiment of this invention comprises a method for fabricating ordered arrays of nanoclusters. The method comprises the steps of using a crystalline mask for low energy electron enhanced etching of nanoscale wells in a substrate, wherein the mask has a crystal structure including an ordered array of nanoscale pores; and depositing additional material to form nanoclusters in the wells. In a preferred embodiment, the mask comprises a protein, more preferably a bacterial S-layer such as an S-layer derived from a member of the genus Sulfolobus. The depositing step may comprise forming a single nanocluster in each of substantially all of the wells.
Another embodiment of the present invention comprises a method for preparing crystalline masks for use in fabricating ordered arrays of nanoclusters on a substrate. The method comprises the steps of providing an isolated crystalline material, wherein the crystal structure includes an ordered array of nanoscale pores; and mounting the crystalline material on the substrate to form a mask for depositing material on the substrate or removing material from the substrate based on the locations of the pores. The providing step may comprise culturing an organism which synthesizes a crystalline material, wherein the crystal structure includes an ordered array of nanoscale pores; and isolating the crystalline material. The mounting step may comprise forming a suspension of the crystalline material in a liquid; applying the suspension to a surface of the substrate; and removing the liquid from the surface. One of the forming step and the applying step may additionally comprise adding a surfactant to the suspension to alter the ability of the liquid to wet the surface.
Yet another embodiment of the present invention comprises a method for fabricating ordered arrays of nanoclusters. The method comprises the steps of using a crystalline mask for performing at least one operation on a substrate, wherein the crystalline mask has a crystal structure which includes nanoscale pores, and the operation is selected from depositing material on the substrate and removing material from the substrate based on the locations of the pores. The mask may be of biological origin, such as from bacteria of the genus Sulfolobus.
In another embodiment of the present invention, a substrate includes an ordered array of nanoclusters, wherein the nanoclusters have a uniform size small enough to allow true quantum confinement behavior, and the nanoclusters occur in a repeating geometric pattern. Preferably, the nanocluster spacing is between about 3 and about 30 nm. Also preferably, the nanoclusters have a diameter varying from about 2 to about 10 nm. The nanoclusters may be formed inside regularly arrayed wells in the substrate, with substantially no lattice displacement adjacent to the wells. Preferably, substantially all of the wells each contain a single nanocluster. The nanoclusters may comprise a material selected from titanium, germanium, and silicon. Further, the substrate may include a plurality of groups of regularly arrayed wells, with the groups arranged in a predetermined pattern.